cephfs: set Pool parameter to empty for Snapshot-backed volumes

Set VolumeOptions.Pool parameter to empty for Snapshot-backed volumes.
This Pool parameter is optional and  only used as 'pool-layout' parameter
during subvolume and subvolume clone create request in cephcsi
and not used for Snapshot-backed volume at all.
It is not saved anywhere for use in subsequent operations after create too.
Therefore, We can set it to empty and not error out.

Signed-off-by: rakshith-r <rar@redhat.com>

internal/cephfs/store/volumeoptions.go

@@ -542,9 +542,10 @@ func (vo *VolumeOptions) populateVolumeOptionsFromBackingSnapshot(
 		return fmtBackingSnapshotOptionMismatch("clusterID", vo.ClusterID, parentBackingSnapVolOpts.ClusterID)
 	}
 
-	if vo.Pool != "" {
-		return errors.New("cannot set pool for snapshot-backed volume")
-	}
+	// Pool parameters is optional and only used to set 'pool_layout' argument for
+	// subvolume and subvolume clone create commands.
+	// Setting this to empty since it is not used with Snapshot-backed volume.
+	vo.Pool = ""
 
 	if vo.MetadataPool != parentBackingSnapVolOpts.MetadataPool {
 		return fmtBackingSnapshotOptionMismatch("MetadataPool", vo.MetadataPool, parentBackingSnapVolOpts.MetadataPool)